In order for a semiconductor device to be used in an electronic product, it must be electrically, mechanically and, in some cases, thermally interconnected to a semiconductor package. Conventionally, mechanical interconnection is achieved with an adhesive material (e.g. a polymer material) in paste or film form. Electrical connection is conventionally achieved by attaching thin gold wires or tabs between the electrical outputs of the semiconductor device and the inputs of the semiconductor package. When thermal conduction is required, such as for power devices, the conventional strategy is to replace the polymer mechanical adhesive with a solder preform that is disposed between the semiconductor device and semiconductor package at the time of assembly.
There is continuous pressure to reduce cost, increase throughput and yield, and increase performance and reliability in packaged semiconductor devices. Simplification of the package is one way to achieve these goals. If a single material, applied onto a wafer rather than to individual die that are singulated (diced) from such a wafer, can be used to create the mechanical, electrical and thermal connection between the die and a package with no compromise in performance, that would be a very advantageous configuration.
Die-attach materials are used to connect semiconductor die to lead-frames, other die, heat sinks and the like. Die-attach materials are usually solders or filled polymeric materials. The fillers may be electrically conductive or non-conductive depending on the requirements of the connection. Early versions of the die-attach material primarily provided a mechanical bond; However, as semiconductor devices have advanced to more complex and powerful uses, they have become more heat-generating during operation, and thus conduction of waste heat, has become a desired features for the die-attach material. Similarly, electrical conduction can be desirable for some applications. Early die-attach materials were primarily applied by dispensing or printing operations; but as component density has increased, achieving a more controlled and consistent bond-line with a minimal fillet, than can be provided by earlier methods, has become increasingly important. Film and wafer-back-coating (WBC) die-attach materials address the challenges posed by component density increases and also reduce the yield losses that are frequently associated with application by dispensing onto singulated die. Film- and WBC-based die-attach materials also significantly improve throughput by replacing a repetitive, die-by-die operation with a single application operation at the wafer level. The primary deficiency of conventional film- and WBC-based die-attach materials is that they do not have particularly high thermal conductivity.
For high power applications, where solder is the die-attach material of choice due to its high thermal conduction, current film and WBC die-attach materials are not a viable solution. Solders, however, have some detrimental characteristics as well. One major drawback to solders is that the only choices that will not remelt during subsequent assembly operations are gold-tin and high-lead solders. Gold-tin is, of course, very expensive and lead is becoming increasingly unacceptable in electronic products due to toxicity. Further, solders have a tendency to form voids in the bond line, both during die bonding and during subsequent thermal excursions. These voids create hot spots of poor thermal conductivity under the die and increase the requirement for post-bonding inspection to ensure acceptable results.
It would therefore be advantageous to have a die-attach material that could be applied at the wafer-level like a film or paste, while having the thermal and electrical performance of a solder die-attach material (e.g. RDS(on) stability through multiple reflow, JEDEC L1 reliability). It would also be advantageous for a wafer-applied die-attach material to provide the superior control of bond-line size and shape that film and WBC die-attach materials offer. Further, it would be advantageous if the wafer-applied die-attach material were lead-free and did not remelt like solder during subsequent assembly steps.
Thus, there remains a need for improved materials and methods that combine the desired properties of both solder and film or WBC die-attach materials. Optimally, such materials could be applied to either or both sides of a wafer by any of a variety of process techniques; could be stored for a period of time after application and suitable initial processing, prior to die singulation and placement; are b-stageable such that after b-staging or initially processing they are substantially tack-free; would maintain a controlled bond-line thickness during die attachment; would not create large fillet areas around the die during die attachment; could be applied in a pattern on the wafer or as a continuous coating; would be lead-free; could be processed at a temperature below 260° C. and would not remelt during subsequent thermal excursions (e.g. during component assembly); and provides high and stable thermal and electrical conductance between the die and the adherend to which it is joined.